1. Field of the Invention
The present invention relates to a motor control device and a motor drive system for controlling the driving of a motor.
2. Description of Related Art
In a motor drive system for driving a permanent-magnet synchronous motor, the angle of the rotor of the motor is often detected by use of an angle sensor. The angle of the rotor is also called the position of the rotor, or the magnetic pole position. The motor drive system incorporates a motor control device, which performs vector control based on the detected angle.
Inconveniently, in the fitting of the angle sensor to the motor, fitting errors are unavoidable. Also, under the influence of temporal changes, the errors can vary in magnitude. Fitting errors produce an error in the detected angle. Motors capable of generating high torques tend to have comparatively large number of pole pairs; in vector control, the larger the number of pole pairs a motor has, the greater the influence of the detection error is (because the larger the number of pole pairs a motor has, the larger the error in terms of electrical angle is). An error in the detected angle may cause unintended torque generation or regeneration charging, possibly causing unintended acceleration, overcharging, and other inconveniences; it also lowers the motor efficiency.
With the aim to overcome such inconveniences, there have conventionally been proposed a number of technologies for correcting the detected angle.
For example, according to one disclosed method, the back electromotive voltage generated when a motor is driven from outside is measured and, based on the back electromotive voltage, the detected angle is corrected.
According to another conventional method, while the rotor is rotating, vector control is performed with the specified d-axis current value and the specified q-axis current value both held at zero and, based on the specified d-axis voltage value and the specified q-axis voltage value during that vector control, the detected-angle correction amount (magnetic pole position correction amount) is found.
According to yet another conventional method, the detected angle is corrected such that the input electric power, which is given as the arithmetic product of the input voltage and the input current to an electric power converter, is equal to the target output, which is calculated from the specified torque and the speed.
Also known is a method according to which, when the load torque is constant, the specified d-axis current value is controlled such that the armature current is minimal and, based on the specified d-axis current value that makes the armature current minimal, the error angle is calculated to correct the detected angle (the rotation position of the magnetic poles). Further known is a method according to which, when a motor is not rotating, it is energized with direct-current power to calculate the angle correction value.
As described above, in cases where vector control is performed by use of an angle sensor, it is necessary to use one or other technology for correcting the detected rotor angle. Disadvantageously, however, conventional correction methods are not satisfactory in terms of the simplicity of configuration and processing, or in terms of practicality. Also, higher accuracy is sought in detected-angle correction.
Moreover, in a case where the fitting errors of an angle sensor is abnormally large, it is highly likely that, unintendedly, a motor generates a comparatively high torque, or regenerated electric power causes overcharging. It is therefore also necessary to use a technology for detecting such abnormalities.